Isolating linear amplifier

ABSTRACT

A multistage electrically isolated, optically coupled, amplifier with balanced interstage forward and feedback coupling. A first pair of optical couplers are balance connected to the output of the first stage and the input of the second stage for forward interstage signal coupling. A second pair of optical couplers are balance connected to the output of the second stage and the input of the first stage for feedback signal coupling.

United States Patent 1 Herbert ISOLATING LINEAR AMPLIFIER [76] Inventor:William L. Herbert, 1405 Inglis Ave, Columbus. Ohio 43212 [22] Filed:Sept. 9, 1973 [2|] Appl. No.: 377,708

[451 July 1,1975

Attorney, Agent, or Firm-Frank H. Foster [5 7] ABSTRACT A multistageelectrically isolated, optically coupled, amplifier with balancedinterstage forward and feedback coupling. A first pair of opticalcouplers are bal ance connected to the output of the first stage and theinput of the second stage for forward interstage signal coupling. Asecond pair of optical couplers are balance connected to the output ofthe second stage and the input of the first stage for feedback signalcoupling.

11 Claims, 4 Drawing Figures [52] US. Cl 330/59; 330/34 [5|] Int. Cl.03F 17/00 [58] Field of Search 330/59, 34; 250/55l, ZlO, 250/209 [56]References Cited UNITED STATES PATENTS 3,327,239 6/1967 Carpenter .i330/59 3,433,962 3/l969 Neiger 330/59 X I F] K, 7 l

ISOLATING LINEAR AMPLIFIER BACKGROUND This invention relates generallyto a multistage isolating amplifier and more particularly relates tosuch an amplifier having improved, balanced, optical coupling in boththe forward and the feedback loops.

The recent development of optically coupled isolators has stimulated thedevelopment of electronic circuits which are electrically isolated andin which the signals are optically coupled. Electrical isolation permitsimproved safety and protection of the humans who are associated with theelectrical equipment.

For example, electronic equipment in a hospital presents increasinghazards for patients and personnel as more complex devices are used forpatient care and monitoring. Electrical interaction can occur betweensuch devices as physiological monitoring and other instruments and alsobetween such instruments and patients or personnel. In cardiacmonitoring systems, for example, it has been determined that onlymicroamps across the heart muscle can produce lethal microshock.

Similarly, workmen dealing with high voltage systems, such as a highvoltage power supply or power transmission lines are quite susceptibleto electrical shock. Electrical isolation of high voltage monitoringinstrumentation could protectively isolate such workmen from thesehazards.

Although electrical isolation is recognized as being desirable forsafety purposes, it is also recognized that electronic circuits andequipment require certain basic characteristic qualities in order toadequately perform. For example, it is necessary that isolatedamplifiers exhibit the same desirable characteristics as sought after indesigning conventional amplifier circuits. For example, suitablelinearity and bandwidth are needed for processing analog signals.

It is therefore an object of the invention to provide an opticallycoupled amplifier system exhibiting high quality linearitycharacteristics from dc to megahertz frequencies.

Another object of the invention is to provide such an amplifier whichfeatures optically coupled negative or positive feedback.

Another object of the invention is to provide such a linear, isolatedamplifier which is readily suitable for reduction to thin film orintegrated circuit construction.

Further objects and features of the invention will be apparent from thefollowing specification and claims when considered in connection withthe accompanying drawings illustrating several embodiments of theinvention.

SUMMARY The invention is a multistage. isolating amplifier ineluding animproved coupling circuit for coupling a signal from the output of afirst amplifier stage to the input of another amplifier stage withelectrical isolation between the amplifier stages. The improvement hasan electrically isolated power supply associated with each amplifierstage. each supply having a common terminal connected to a commonterminal of its associated amplifier stage and having a relativelypositive terminal and a relatively negative terminal.

The coupling means comprises a pair of optical couplers having theirphoto emissive input element in series balanced connection between therelatively posi tive and negative supply terminals associated with saidone amplifier stage for permitting a quiescent current flow through theinput elements and having the output of the one amplifier stageconnected intermediate the input elements. The optical couplers alsohave their photo responsive elements in series balanced connectionbetween the relatively positive and negative terminals of the powersupply associated with the other amplifier stage for permitting aquiescent current flow through said output elements and having the inputof said other amplifier stages connected intermediate the outputelements.

DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of arelatively simple preferred embodiment of the invention.

FIG. 2 is a closed loop block diagram illustrating the transferfunctions of the preferred embodiment of the invention.

FIG. 3 is a schematic diagram ofa more complex embodiment of theinvention.

FIG. 4 is a schematic diagram of an alternative inverting embodiment ofthe invention.

In describing the preferred embodiment of the inven' tion illustrated inthe drawings, specific terminology will be resorted to for the sake ofclarity. However it is not intended to be limited to the specific termsso selected and it is to be understood that each specific term includesall technical equivalents which operate in a similar manner toaccomplish a similar purpose. For example, it is very important that theterm connection is not limited to direct connection but may includeconnection through other elements such as resistors where such aconnection would be considered equivalent for purposes of the inventionby those skilled in the art.

DETAILED DESCRIPTION FIG. 1 illustrates a multistage, isolating, linearamplifier constructed according to the present invention. This amplifierhas a first or prior stage I followed by a second or subsequent stage 2.Each stage includes an op-amp 4 and 6 conventionally connected withtheir noninverting inputs connected to a common and with feedbackresistances R and R respectively in order to operate as conventionallinear amplifiers.

An electrically isolated power supply is associated with each amplifierstage. Each such power supply has a common terminal connected to acommon terminal of its associated amplifier stage and has both arelatively positive terminal and a relatively negative terminal.

For example, the first stage 1 is provided with a negatively groundedbattery I0 having its positive terminal 12 forming the relativelypositive terminal of the first stage 1. Similarly, the first stage I hasanother battery 14 positively grounded and having its negative terminal16 forming the relatively negative terminal of the first stage I. In alike manner. the batteries I8 and 20 pro vide the relatively positiveterminal 22 and the relatively negative terminal 24 for the second stage2. As can be seen from the schematic, the op-amps themselves are alsoconventionally supplied by these same power supplies.

Generally described. the output of the first stage 1 is forward coupledto the input of the second stage 2 by a pair of optical couplers havingtheir photo emissive input elements in series balanced connectionbetween the relatively positive terminal l2 and the relatively negativeterminal 16 associated with the first stage 1 for permitting a quiescentcurrent flow through these input elements. The output terminal 30 of theamplifier 4 is connected intermediate these input elements. The sameoptical couplers also have their photo responsive output elements inseries balanced connection between the relatively positive tenninal 22and the relatively negative terminal 24 of the power supply associatedwith the second stage 2 for permitting a quiescent current flow throughthe output elements and having the input terminal 32 of the secondamplifier 6 connected intermediate the output elements. A feedback pathfrom the output of the second stage 2 to the input of the first stage iis provided by a similar coupling means which comprises a similarlyconnected pair of optical couplers.

More specifically, a light emitting diode 40 is optically coupled to aphoto transistor 42 to form one optical coupler of the forward couplingmeans. Similarly, a light emitting diode 44 is optically coupled to aphoto transistor 46 to form the other optical coupler of the forwardcoupling means. A pair of biasing resistances 50 and 52 are seriesconnected with the light emitting diodes 42 and 44 and the outputterminal 30 of the amplifier 4 is connected therebetween.

For purposes of the present invention, the biasing resistances for eachamplifier stage are considered to be a part of the coupler because theparticular biasing arrangement is dependant in part upon the operatingcharacteristics of the particular optical coupler being used and thebias desired. As will be seen the coupler is always in balancedconnection to the amplifier stage. The load seen by the opamps thereforeis the impedance presented by the entire coupler circuit connected toit.

Phototransistors 42 and 46 are in similar balanced connection with therelatively positive terminal 22 and the relatively negative terminal 24of the second stage 2. The input 32 of the amplifier 6 is connectedintermediate the phototransistors 42 and 46.

Similarly, light emitting diodes 60 and 62 are optically coupled tophototransistors 64 and 66 respec tively for coupling a feedback signalfrom the output 68 of the amplifier 6 to the inverting input 70 of theamplitier 4. This feedback coupling path includes biasing resistances 72and 74 which are analogous to the biasing resistances 50 and 52 of thefirst stage. The signal input terminal 75 of the first stage 1 isconnected by an input resistance Rn to the inverting input 70 of theop-amp 4.

For purposes of the present invention the term optical coupler is notlimited to devices which are coupled by light in the visible range butis intended to include other devices having similar characteristics. Forexample, the term optical coupler is intended to include devicesoperating in the infra-red region. An optical cou pler is considered toinherently include a photo emissive element which emits light of somefrequency and a photo responsive element optically coupled to the photoemissive element which responds to the emitted light.

The operation of the preferred embodiment of the invention illustratedin FIG. 1 may first be considered with the circuit in a balancedquiescent condition. ln such a condition, the input terminal 75 to theamplifier 4 may be considered to have applied thereto 0 volts, i.e., itis grounded. In this condition the output 30 of the op-amp 4 will be atground potential and the quiescent current i,., through the lightemitting diode 40 and the associated biasing resistance 50 will equaltl..- quiescent current i through the light emitting diode 44 and thebiasing resistance 52. Therefore, the circuit is in balance and therewill be substantially no output current 1' at the output of theamplifier 4.

[n this quiescent condition the light emitted from light emitting diodes40 and 44 will be equal and consequently, the collector to emittercirrent i and i of the balance connected phototransistors 42 and 46willl be equal. The input current is balanced and the voltage at theinput 32 of op-amp 6 will be ground potential.

The input voltage of the amplifier 6 will consequently be zero andtherefore its output potential at the output terminal 68 will be atground potential. Therefore the currents i and i through the lightemitting diodes 60 and 62 under these quiescent conditions will beequal. This in turn will cause equal output collector to emittercurrents i and i in the collector circuits of the phototransistors 64and 66. This results in a balanced, ground potential condition at theinput of the amplifier 4.

Therefore, it can be seen that under quiescent conditions, the entirecircuit is balanced with the inputs and outputs to all stages resting atground potential.

If now, we consider the attachment of a small signal generator at theinput terminal of the first stage 1, we can consider the result ofvoltage excursions about this ground potential. A positive voltageexcursion at the input 75 will cause a negative going excursion of theoutput 30 of the amplifier 4. This will produce an increase of currenti, through light emitting diode 40 and a decrease of current i throughlight emitting diode 44. The difference in current i. and i will besubstantially the current i at the op-amp 4 output 30 according toKirchhoffs current law.

The resulting current imbalance in the light emitting diodes 40 and 44will produce a corresponding imbalance in the collector circuit currentsi. and i of the phototransistors 42 and 46 so that the current i willbecome significantly greater than the current The difference currentwill flow primarily through the resistor R (with only a relativelyinsignificant current input current to the op-amp 6). The impedanceimbal ance will cause enough voltage imbalance of the input 32 of op-amp6 to cause a positive going voltage excursion at the output 68 of theamplifier 6. Therefore the output is in phase with the input.

The positive going excursion of the output 68 simi larly increases thecurrent i,- in the light emitting diode 62 while decreasing the currenti}; through the light emitting diode 60. This in turn reduces theimpedance of phototransistor 66 while increasing the impedance ofphototransistor 64 producing a net imbalance of the feedback circuitdirected oppositely of the original input encrrsion at the inputterminal 75. The circuit consequently exhibits negative feedback.Clearly, positive feet ck could be provided if the light emitting diode66 were optically coupled to phototransistor 66 with the light emittingdiode 62 optically coupled to phototransistor 64.

For a negative going excursion, the circuit becomes imbalanced in theopposite directions with analogous results occurring.

FIG. 2 represents the embodiment of FIG. 1 in a conventional closed loopblock diagram for determining the small signal, low frequency operationof this circuit. The biasing resistance value R must of course, besubstantially the parallel combination of resistances 50 and 52 as isappropriate for the AC equivalent circuit. Similarly, the biasingresistance R will be the parallel combination of resistances 72 and 74.The term CTR represents the coupler transfer ratio and for the couplersshown would be the collector current transfer ratio which may be definedas the ratio of the collector current to the input diode current.

An amplifier constructed according to the present invention can includeseveral stages such as those illustrated in FIG. I. Additionally,feedback coupling could be provided between any subsequent stage and anyprior stage. For example, there could be four cascaded stages coupled bythe coupling means of the present invention in the forward directionwith feedback coupling according to the present invention from the laststage to the first stage. Additionally, or alternatively, feedbackcoupling could be provided between intermediate stages.

There are many equivalent combinations of optical couplers which can beused in an embodiment of the invention. For example, both thephotoemissive element and the photoresponsive element of each couplercould be a diode. This would be advantageous for high frequencyapplications. Additionally, the input photoemissive elements could bedriven by a conventional transistor and similarly the photoresponsiveoutput elements could be used for controlling a conventional transistorfor improving the gain of the coupling circuitry. The coupling circuitrycould also include incandescent or other types of photoemissive circuitelements and photoresistors or other types of photoresponsive elements.Although there are a great multitude of such circuits which will nowbecome apparent to those skilled in the art it is intended for thepurposes of the present invention that any such circuit together withits biasing arrangements and loading arrangements for the amplifierstages, be considered, as a whole, a coupling circuit.

Optical couplers of many types are available for use with the presentinvention. For example, the Monsanto photodiode optoisolator MCDZ may beused where a photodiode optically coupled to a light emitting diode isdesired. Alternatively, the Fairchild optically coupled isolator FCD8I lor FCD820 provides an infrared emitting diode optically intercoupled toa phototransistor.

The present invention also lends itself well to optical coupling throughvarious types of light paths including air or vacuum and is particularlywell suited for the coupling now known in the field of fiber optics.

By selection of the forward and feedback coupling paths the output canbe inverted or non-inverted and for each case either negative orpositive feedback can be provided.

It should also be noted that the amplifier circuit within each stage canin fact be any amplifier, including conventional amplifiers, which cancause light emission variations in light emitting devices. Ofcourse, aswill be obvious to those skilled in the art, the amplifier itself withineach stage could be a multiple stage amplifier having its own feedbacksystem according to conventional practice.

FIG. 3 illustrates an alternative embodiment of the invention whichcontains several alternatively available features. For example, lightemitting diodes and 82 are driven by complementary transistors 84 and86. Similarly, photoresponsive diodes 88 and 90 are used for controllingcomplementary transistors 92 and 94. The use of such transistors permitsthe high frequency response available from diodes and yet improves thecoupled gain.

The embodiment of FIG. 3 also has an impedance matching amplifiercircuit 96 utilized to provide a high impedance input terminal 98. Inorder that the quiescent output voltage at the output terminal 100 canbe adjusted to zero under quiescent conditions, an adjustable biasingmeans 102 is provided for selectively adjusting the output 100 to groundpotential or any other desired output condition with the input terminal98 connected to ground potential. The preferred adjustable biasing meanscomprises an adjustable voltage divider which is connected between therelatively negative and relatively positive terminals and 112 and hasits movable wiper I14 resistively connected to an intermediate node ofone of the coupling means. In FIG. 3, the movable wiper 114 is connectedto the node intermediate the photoresponsive element 122 and thephotoresponsive element 124 of the feedback coupling means. It can beseen that any one of these alternative modifications or groups of suchmodifications may be adapted for use in the various types of circuitrysuggested by this description of the preferred embodiments of theinvention.

FIG. 4 is a relatively simple circuit and is included as an example ofan embodiment of the invention in which the output is inverted from itsinput phase.

It is to be understood however, that while the detailed drawings andspecific examples given describe preferred embodiments of the invention,they are for the purposes of illustration only, that the apparatus ofthe invention is not limited to the precise details and conditionsdisclosed and that various changes, including those described may bemade therein without departing from the spirit of the invention which isdefined by the following claims.

What is claimed is:

l. A multistage isolating amplifier including an improved circuit forapplying a signal from the output of a first amplifier stage to theinput of a second amplifier stage with electrical isolation between saidamplifier stages, wherein the improvement comprises:

a. at least two electrically isolated power supplies, a different supplyconnected to said first amplifier stage and said second amplifier stagerespectively, each supply having a common terminal connected to a commonterminal of its respective amplifier stage and having a positiveterminal and a negative terminal; and

b. a signal applying means for applying a signal from the output of saidfirst amplifier stage to the input of said second amplifier stage, saidsignal applying means comprising a pair of optical couplers having theirphotoemissive elements in series connection between the positive andnegative supply terminals of the power supply connected to said firstamplifier stage for permitting a quiescent current flow through saidphotoemissive elements and having the output of said first amplifierstage connected intermediate said photoemissive elements, said op ticalcouplers also having their photoresponsive elof an amplifier stage whichis subsequent to said first amplifier stage, said first stage and saidsubsequent stage each having a respective power supply,

emissive elements in series connection between the positive and negativeterminals of the epower supply connected to said one amplifier stage forpermitting a quiescent current flow through said phoements in seriesconnection between the positive toemissive elements of said second pairof optical and negative terminals of the power supply concouplers andhaving the output of said one amplinected to said second amplifier stagefor permitting fier stage connected intermediate said photoemisaquiescent current flow through said photoresponsive elements of saidsecond pair of optical cousive elements and having the input of saidsecond plers, said second pair of optical couplers also havamplifierstage connected intermediate said photo It) ing their photoresponsiveelements in series con responsive elements. nection between the positiveand negative termi- 2. An improved multistage isolating amplifier havingnals of the power supply connected to said prior multiple cascadedamplifier stages and including: amplifier stage for permitting aquiescent current a. a forward signal coupling means for coupling theflow through said photoresponsive elements of said output signal of afirst amplifier stage to the input second pair of optical couplers andhaving the input of said prior amplifier stage connected intermediatesaid photoresponsive elements of said second pair of optical couplers.

each power supply having a common terminal con- 3. An amplifieraccording to claim 2 wherein said nected to a common terminal of itsrespective amamplifier stages each comprises an op-amp havingbiasplifier stage and each power supply having a posiing means wherein,under quiescent conditions, the tive terminal and a negative terminal,said coupling outputs and inputs of said op-amps are substantially atmeans comprising a first pair of optical couplers a ground potentialintermediate the potentials of said having their photoemissive elementsin series conrelatively positive and negative tenninals and havesubnection between the positive and negative termistantially no input oroutput current.

nals of the power supply connected to said first am- 4. An amplifieraccording to claim 3 wherein an ad plifier stage for permitting aquiescent current flow justable biasing means is provided forselectively adthrough said photoemissive elements of said first justingsaid amplifier to a desired quiescent output conpair of optical couplersand having the output of dition.

said first amplifier stage connected intermediate 5. An amplifieraccording to claim 4 wherein said adsaid photoemissive elements of saidfirst pair of opjustable biasing means comprises an adjustable voltagetical couplers, said first pair of optical couplers also dividerconnected across said relatively negative and having theirphotoresponsive elements in series positive terminals and having itsmovable member reconnection between the positive and negativetersistively connected to an intermediate node of one of minals of thepower supply connected to said sub said coupling means.

sequent amplifier stage for permitting a quiescent 6. An amplifieraccording to claim 2 wherein the opcurrent flow through saidphotoresponsive eletical couplers of at least one coupling means eachcomments of said first pair of optical couplers and havprises a lightemitting diode coupled to a photo transising the input of saidsubsequent amplifier stage tor.

connected intermediate said photoresponsive ele- 7. An amplifieraccording to claim 6 wherein said ments of said first pair of opticalcouplers; and light emitting diode is driven by a transistor.

b. a feedback circuit means for applying the output 8. An amplifieraccording to claim 2 wherein the opsignal of one of the amplifier stagesof said multitical couplers of at least one of said coupling means stageamplifier to the input of an amplifier stage comprises a light emittingdiode and a photo diode. which is prior to said one amplifier stage,said one 9. An amplifier according to claim 8 wherein said amplifierstage and said prior stage each having a light emitting diode is drivenby a transistor. respective power supply, each power supply having 10.An amplifier according to claim 8 wherein said a common terminalconnected to a common termiphoto diode is connected to a secondtransistor for connal of its respective amplifier stage and each powertrolling said second transistor. supply having a positive terminal and anegative 11. An amplifier according to claim 10 wherein said terminal.said feedback circuit means comprising a light emitting diode is drivenby a transistor. second pair of optical couplers having their photo-

1. A multistage isolating amplifier including an improved circuit forapplying a signal from the output of a first amplifier stage to theinput of a second amplifier stage with electrical isolation between saidamplifier stages, wherein the improvement comprises: a. at least twoelectrically isolated power supplies, a different supply connected tosaid first amplifier stage and said second amplifier stage respectively,each supply having a common terminal connected to a common terminal ofits respective amplifier stage and having a positive terminal and anegative terminal; and b. a signal applying means for applying a signalfrom the output of said first amplifier stage to the input of saidsecond amplifier stage, said signal applying means comprising a pair ofoptical couplers having their photoemissive elements in seriesconnection between the positive and negative supply terminals of thepower supply connected to said first amplifier stage for permitting aquiescent current flow through said photoemissive elements and havingthe output of said first amplifier stage connected intermediate saidphotoemissive elements, said optical couplers also having theirphotoresponsive elements in series connection between the positive andnegative terminals of the power supply connected to said secondamplifier stage for permitting a quiescent current flow through saidphotoresponsive elements and having the input of said second amplifierstage connected intermediate said photoresponsive elements.
 2. Animproved multistage isolating amplifier having multiple cascadedamplifier stages and including: a. a forward signal coupling means forcoupling the output signal of a first amplifier stage to the input of anamplifier stage which is subsequent to said first amplifier stage, saidfirst stage and said subsequent stage each having a respective powersupply, each power supply having a common terminal connected to a commonterminal of its respective amplifier stage and each power supply havinga positive terminal and a negative terminal, said coupling meanscomprising a first pair of optical couplers having their photoemissiveelements in series connection between the positive and negativeterminals of the power supply connected to said first amplifier stagefor permitting a quiescent current flow through said photoemissiveelements of said first pair of optical couplers and having the output ofsaid first amplifier stage connected intermediate said photoemissiveelements of said first pair of optical couplers, said first pair ofoptical couplers also having their photoresponsive elements in seriesconnection between the positive and negative terminals of the powersupply connected to said subsequent amplifier stage for permitting aquiescent current flow through said photoresponsive elements of saidfirst pair of optical couplers and having the input of said subsequentamplifier stage connected intermediate said photoresponsive elements ofsaid first pair of optical couplers; and b. a feedback circuit means forapplying the output signal of one of the amplifier stages of saidmultistage amplifier to the input of an amplifier stage which is priorto said one amplifier stage, said one amplifier stage and said priorstage each having a respective power supply, each power supply having acommon terminal connected to a common terminal of its respectiveamplifier stage and each power supply having a positive terminal and anegative terminal, said feedback circuit means comprising a second pairof optical couplers having their photoemissive elements in seriesconnection between the positive and negative terminals of the epowersupply connected to said one amplifier stage for permitting a quiescentcurrent flow through said photoemissive elements of said second pair ofoptIcal couplers and having the output of said one amplifier stageconnected intermediate said photoemissive elements of said second pairof optical couplers, said second pair of optical couplers also havingtheir photoresponsive elements in series connection between the positiveand negative terminals of the power supply connected to said prioramplifier stage for permitting a quiescent current flow through saidphotoresponsive elements of said second pair of optical couplers andhaving the input of said prior amplifier stage connected intermediatesaid photoresponsive elements of said second pair of optical couplers.3. An amplifier according to claim 2 wherein said amplifier stages eachcomprises an op-amp having biasing means wherein, under quiescentconditions, the outputs and inputs of said op-amps are substantially ata ground potential intermediate the potentials of said relativelypositive and negative terminals and have substantially no input oroutput current.
 4. An amplifier according to claim 3 wherein anadjustable biasing means is provided for selectively adjusting saidamplifier to a desired quiescent output condition.
 5. An amplifieraccording to claim 4 wherein said adjustable biasing means comprises anadjustable voltage divider connected across said relatively negative andpositive terminals and having its movable member resistively connectedto an intermediate node of one of said coupling means.
 6. An amplifieraccording to claim 2 wherein the optical couplers of at least onecoupling means each comprises a light emitting diode coupled to a phototransistor.
 7. An amplifier according to claim 6 wherein said lightemitting diode is driven by a transistor.
 8. An amplifier according toclaim 2 wherein the optical couplers of at least one of said couplingmeans comprises a light emitting diode and a photo diode.
 9. Anamplifier according to claim 8 wherein said light emitting diode isdriven by a transistor.
 10. An amplifier according to claim 8 whereinsaid photo diode is connected to a second transistor for controllingsaid second transistor.
 11. An amplifier according to claim 10 whereinsaid light emitting diode is driven by a transistor.